The present invention relates to extrusion dies for the continuous extrusion of fine honeycomb structures from plasticized inorganic powder mixtures, and more particularly to honeycomb extrusion dies comprising improved discharge slot surface finish profiles that impart more uniform and stable extrusion characteristics to the dies.
The manufacture of inorganic honeycomb structures from plasticized powder batches comprising mineral or other inorganic powders dispersed in appropriate binders is well known. U.S. Pat. Nos. 3,790,654, 3,885,977, and 3,905,743 describe dies, processes and compositions for such manufacture, while U.S. Pat. Nos. 4,992,233 and 5,011,529 describe honeycombs of similar cellular structure extruded from batches incorporating metal powders.
The manufacture of extrusion dies for the production of ceramic honeycombs by these methods requires extremely precise machining. To supply material to the die for forming the honeycomb product, the inlet or supply face of the die is provided with multiple apertures or feed holes into which the plasticized batch material to be extruded is forced under high pressure.
The opposing discharge or outlet face of the die is provided with a criss-crossing array of finely machined discharge slots, these slots being cut into the discharge section of the die so that they intersect the holes of the feed hole array. These slots operate to finally shape the plasticized batch supplied through the feed holes into the interconnecting wall structure of the extruded honeycomb.
The islands of material between the intersecting discharge slots that together form the outlet face of the die are sometimes referred to as "pins", since due to the removal of the slot material they appear as free-standing metal posts extending outwardly from the die interior. The cross-sectional end shapes of these pins, which are attached to the die body only at their bases, define the shapes of the honeycomb channels formed by the extruding plasticized batch.
A number of techniques have been employed to form the intersecting discharge slots and pins in the discharge surfaces of metal honeycomb extrusion dies. For mild steels, the discharge slots may be formed by sawing. However, if the die is to be cut from a harder, slower wearing material such as stainless steel, electrical discharge machining or abrasive wheel slitting are more widely used.
In the electrical discharge machining (EDM) process, the discharge slots are formed through an electrical discharge maintained between a long, thin, traveling electrode wire and the metal die preform. U.S. Pat. Nos. 2,526,423, 4,205,213, 4,233,486, 4,403,131 and 4,527,035 provide detailed descriptions of EDM processing. Slot lengths formed by wire EDM in these dies can be 31/2 inches in length or more, and slot depths greater than 0.1 inches with slot widths of 0.012 inches or less can be provided. In general, slots formed in stainless steels by EDM methods are substantially free of burrs and have a relatively smooth and consistent surface finish.
The abrasive wheel slitting process typically involves the use of a diamond or other abrasive-impregnated cutting wheel to slice the narrow discharge slots into the face of the die. Abrasive wheel slitting produces discharge slots of substantially smoother surface finish than even EDM slotting, and has the further advantage that it is not subject to process disruptions from variations in electrical discharge conditions. Slots produced by this process should theoretically reduce slot flow rate variability to insignificant levels.
However, while both of these machining processes have been adapted to the commercial production of honeycomb extrusion dies, neither produces dies with completely satisfactory extrusion performance. Remaining problems, even with abrasive wheel slitting, include uneven flow effects such as "fast flow" (accelerated extrusion through localized regions of a die), which can produce a variety of honeycomb product defects. Such defects include "swollen" or "rippled" webs wherein the cell walls of the honeycomb are thickened, or are wavy in transverse and/or longitudinal directions, due to the local extrusion of excess material at one or more apparently random locations across the die outlet face. Other variations in cell dimensions, cell shapes, and cell wall thicknesses can also be produced.
In many cases surface finish irregularities within the feed hole sections of the extrusion dies are considered to be major contributors to uneven flow. In other instances variations in extrusion batch viscosity are thought to play a role.
Regardless of the cause of such defects, maintaining exact geometry in extruded honeycomb products has become increasingly difficult as the demand for honeycomb products of finer and finer channel wall structure and increased channel count or cell density has increased. For this reason, there remains a critical need to improve both the designs and the machining techniques used for extrusion die fabrication, particularly for thin-walled, high-cell-density products, so that these products can be manufactured at an economical cost.